It is well known that aromatic hydrocarbons having at least one and preferably two or more oxidizable substituent groups may be converted into carboxylic acid products by effecting oxidation of such groups with molecular oxygen under controlled conditions. Such conditions have generally included the use of a known oxidation catalyst together with a suitable solvent.
During the present commercial production of aromatic acids such as terephthalic acid, it is essential that reactor oxygen partial pressure in the oxidation of an alkylaromatic be high enough to prevent oxygen starvation. A high oxygen partial pressure reduces the formation of undesirable colored by-products by suppressing coupling reactions. Also, a high partial pressure increases oxidation reaction rates, which allows higher reactor throughputs, and reduces the burning of the reaction solvent. However, in a commercial operation of such an oxidation system a significant loss of oxidation capacity occurs as a result of insufficient utilization of molecular, oxygen. It is therefore highly desirable to improve utilization of the oxygen and thereby to improve process efficiency and debottleneck and increase the rated capacity of an aforesaid commercial oxidation system and simultaneously to maintain the high quality of the carboxylic acid products produced.
Spillar et al., U.S. Pat. No. 2,962,361 (Nov. 29, 1960) discloses a stagewise continuous countercurrent oxidation system that “enables practically quantitative oxygen utilization . . . without substantial detriment to product yield or quality.” The highest oxygen concentration is introduced at the final stage, and the off-gases from each stage are returned to the preceding stage while the partially oxidized products move from the first stage to the final stage. It is disclosed as desirable that the final oxidation stage be at the highest temperature, pressure and oxygen concentration. It is also disclosed with regard to the first oxidation stage 11 and the vent line 26 from it in FIG. 2 that “additional air or oxygen may be introduced through line 41 . . . for preventing the oxygen concentration in receiver 24, condenser 23 or line 26 from exceeding 8 volume percent (it is preferably zero).” Baldwin et al., U.S. Pat. No. 3,092,658 (Jun. 4, 1963) discloses a stagewise continuous countercurrent oxidation system that is very similar to that in aforesaid U.S. Pat. No. 2,962,361.
Baldwin, U.S. Pat. No. 3,064,044 (Nov. 13, 1962) also discloses a staged-countercurrent oxidation system. The uncondensed off-gases leaving the second oxidation stage are returned to the first oxidation stage, and the patent states should contain less than 8 percent oxygen but it may contain about 1 to 8 percent oxygen and hence it is introduced by lines 14 and 15 to supply oxygen in vessel 11. With regard to the first oxidation stage 11 and the condenser 20 and receiver 21 through which its off-gases pass, the patent also states that “the amount of additional air introduced from line 15 should be controlled so that the oxygen content of gases in condenser 20 and receiver 21 will be less than 8 percent, preferably near zero.”
June et al., U.S. Pat. No. 6,153,790 (Nov. 28, 2000) discloses a process for producing diacid substituted aromatics with a purity of at least 97%. The process comprises contacting in a stirred tank reactor a dialkyl substituted aromatic in an organic acid solvent with an oxidant containing at least 50% by volume of oxygen at an oxygen partial pressure of at least 1 psia, at a temperature between about 176° F. and about 266° F., in the presence of a catalyst system comprising zirconium and cobalt. A vapor stream comprising the organic acid solvent, water vapor and unreacted oxidant is withdrawn from the reactor. More than 50% by volume of oxygen in the oxidant is required so that the total pressure of the reaction system can be low enough to allow reflux cooling of the reaction system at temperatures between about 176° F. and about 266° F. as a result of vaporization of liquid phase components to form the aforesaid vapor stream. The reactor design must effectively provide for nearly complete oxygen consumption below the liquid/gas interface. Nitrogen can be introduced in the vicinity of the liquid/gas interface in a quantity sufficient to render the vapor phase gas mixture nonflammable. The patent discloses that, if desired, after dilution with nitrogen, the unconsumed oxygen can be contacted with feed streams in an optional pre-reactor to provide nearly or fully complete utilization of oxygen.
Turner and Hously, U.S. patent application No. U.S. 2001/0007910 A1 published Jul. 12, 2001; PCT/US01/20109 published Jul. 18, 2002 as WO 02/055468 A1; PCT/US01/00825 published Jul. 19, 2001 as WO 01/51442 A2; and PCT/US01/19960 published Jul. 18, 2002 as WO 02/055467, disclose a process for the staged catalytic liquid phase, air oxidation of a suitable precursor, such as paraxylene, to a carboxylic acid, such as terephthalic acid, comprising oxygenating a feed stream comprising acetic acid and an oxidation catalyst at an elevated pressure of from 2000 up to 20,000 kPa, continuously and simultaneously feeding the oxygenated feed stream and paraxylene to a first reaction zone that is positioned upstream from a conventional oxidation reactor to form a reaction medium in which the acetic acid to paraxylene mass ratio is in the range of from 10:1 to 20:1 and the reaction products are maintained in solution. In this first reactor the uptake of oxygen within the reaction medium in the first reaction zone is limited to less than 50% of the oxygen for full conversion of the paraxylene present to terephthalic acid. Thereafter, the reaction medium is fed from the first oxidation zone to the aforesaid conventional oxidation reactor and simultaneously the pressure of the reaction medium is reduced to a pressure in the range of from 1000 to 2,000 kPa in the conventional oxidation reactor. In WO 01/51442 A2 the process is disclosed as being a method for increasing the production capacity of a conventional oxidation reactor, while the three other patent publications disclose it as a method for reducing the formation of impurities in the final carboxylic acid product and for controlling degradation of the solvent and precursor.
Although it is highly desirable to maximize the utilization of oxygen and thereby improve process efficiency and increase the rated capacity of a commercial oxidation system while maintaining the high quality of the carboxylic acid products produced and without the need of adding additional compressor capacity, that goal has never been achieved and the means for achieving it has never been disclosed.